This invention relates to a method of and apparatus for the separation of air.
The separation of air by rectification is very well known indeed. Rectification is a method in which mass exchange is effected between a descending stream of liquid and an ascending stream of vapour such that the ascending stream of vapour is enriched in a more volatile component (nitrogen) of the mixture to be separated and the descending stream of liquid is enriched in a less volatile component (oxygen) of the mixture to be separated.
It is known to separate air in a double rectification column comprising a higher pressure rectification column which receives a stream of purified, compressed, vaporous air at a temperature suitable for its separation by rectification, and a lower pressure rectification column which receives a stream of oxygen-enriched liquid air for separation from the higher pressure rectification column, and which is in heat exchange relationship with the higher pressure rectification column through a condenser-reboiler, of which the condenser provides liquid nitrogen reflux for the separation and the reboiler provides an upward flow of nitrogen vapour in the lower pressure rectification column.
The double rectification column may be operated so as to produce an oxygen fraction at the bottom of the lower pressure column and a nitrogen fraction at the top of the lower pressure column. The oxygen fraction may be essentially pure, containing less than 0.5% by volume of impurities, or may be impure containing up to 50% by volume of impurities.
There is a net requirement for refrigeration to be provided to the air separation plant. At least part of this requirement arises from the operation of the double rectification column at cryogenic temperatures. Particularly if none of the products of the air separation is taken in liquid state, the requirements for refrigeration are typically met by raising the pressure of a part of the air to at least 2 bar above the operating pressure at the top of the higher pressure column and expanding it with the performance of external work in an expansion turbine which exhausts into the lower pressure column. Typically, the turbine is coupled to a booster-compressor which raises the pressure of the air to above that at the top of the higher pressure column.
An air separation plant typically consumes a considerable amount of power. Its is therefore desirable for the air separation plant to have a configuration which enables power consumption to be minimised without unduly increasing its capital cost. In order to minimise the power consumption much attention in the art has recently been focused upon operating the lower pressure rectification column with two reboilers, one operating at a higher temperature and being heated by a flow of the air to be separated, and the other operating at a lower temperature and being heated by a flow of nitrogen separated in the higher pressure rectification column. A disadvantage of such plant is that the requirement for a second reboiler adds to its capital cost.
U.S. Pat. No. 5,337,570 provides examples of a yet further kind of air separation plant. There is a first condenser-reboiler which condenses a part of the top nitrogen fraction separated in the higher pressure column. The condensation is effected by indirect heat exchange with a stream of a bottom oxygen-enriched liquid fraction formed in the higher pressure column. As a result, the stream of the bottom oxygen-enriched liquid fraction is partially reboiled. Resulting vapour and residual liquid are fed to the lower pressure rectification column. The plant employs a single generator-loaded expansion turbine exhausting into the lower pressure column. The air to be separated is compressed in a main, plural stage, compressor. The main air feed to the higher pressure rectification column is taken from a lower pressure stage than the feed to the expansion turbine.
It is an aim of the present invention to provide a method and apparatus for separating air by rectification which are able to be operated at a favourable net power consumption without imposing on the apparatus an unacceptably high capital cost and without the need to have two reboilers associated with the lower pressure rectification column.